1. Field of the Invention
The present invention relates to an imaging lens for a small imaging apparatus using a solid-state image sensing element, such as an image sensing element of a charge-coupled device (CCD) type or a complementary mental-oxide semiconductor (CMOS) type.
2. Description of the Related Art
In many cases, an imaging lens used for a cellular phone or the like has been manufactured by injection molding with thermoplastic resin in view of mass production and cost. Also, in recent years, to simplify the manufacturing process and to reduce the cost, a lens module has been required to resist a reflow process. In particular, an imaging lens that resists a solder reflow process has been demanded. There is such a demand; however, a thermoplastic resin lens cannot resist the temperature of the solder reflow process. Hence, a heat-resistant imaging lens formed by various methods is being gradually suggested.
Japanese Patent No. 3926380 discloses, as an example of a highly heat-resistant lens, a junction type compound lens that is one type of hybrid lens in which optical glass is combined with heat-resistant UV-curable resin or thermosetting resin. The junction type compound lens addresses the problem of heat resistance and also the problem of interface reflection of the junction type compound lens by controlling the difference between refractive indices of the glass and the resin material to be a constant value or smaller.
Further, in order to reduce chromatic aberration without using a diffraction surface, Japanese Patent No. 4293291 discloses a method of constituting a junction type compound lens by controlling Abbe numbers of lenses that are formed on both surfaces of a transparent plane parallel plate to be a predetermined value.
As described above, the junction type compound lens is gradually being used in a situation with a difficulty in application of a resin lens, such as when the lens is subjected to, for example, solder reflowing, by combining heat-resistant energy-curable resin with glass.
A glass lens is not changed in shape or deteriorated in performance at the temperature of the solder reflow process. However, it is known that, if an aspherical lens is formed of a glass material, the aspherical lens which is frequently used for reducing various aberrations, the cost may become very high. If an aspherical lens is to be formed of glass, a method called aspherical-surface glass molding is widely performed as a method available for mass production. This method manufactures an aspherical glass lens by using a low-melting-point glass and a die; however, the cost of this method is higher than the cost of a plastic lens obtained by a conventional injection molding method or a junction type compound lens that uses glass and curable resin. In the glass molding method, a precise preform, the volume of which is controlled and the shape of which is relatively close to a final aspherical surface shape, is set on a die that is heated at a deformation point of glass or higher, the preform is pressed by another die, and hence an aspherical surface is formed on both surfaces or one surface. The precise preform with the controlled volume is expensive, and the life of the dies that are exposed to the high temperature is short. For the mass production, multiple dies have to be prepared. Owing to this, it is difficult to supply lenses with a low cost.
Since the aspherical surface portion of the junction type compound lens is formed of the curable resin, the degree of freedom of the shape is high. Also, the temperature of a die is around a room temperature if UV-curable resin is used, and the temperature of a die is about 200° C. if thermosetting resin is used. Hence, the process is at a temperature that is markedly lower than 400° C. or higher of the temperature of the step in the glass molding method. Therefore, the life of the die is long. In addition, the process time is from about several seconds to about two minutes in case of the UV-curable resin, and is from about one to about ten minutes in case of the thermosetting resin. Hence, a tact time (a cycle time) is shorter than that of the glass molding method. Further, the heat-resistant UV-curable resin and thermosetting resin are generally expensive. In contrast, since the junction type compound lens uses a spherical glass or a plane parallel glass plate, the volume of resin to be used is small. Therefore, with the junction type compound lens, a heat-resistant lens that can resist the temperature of the solder reflowing can be provided with a low cost.
Meanwhile, it is known that reflection occurs at interfaces between the glass and resin of the junction type compound lens in accordance with the difference between the refractive indices of the glass and resin. For example, in case of a junction type compound lens using a resin with a refractive index of 1.4 and a glass with a refractive index of 1.6, when a ray is perpendicularly incident on the interfaces, the ray is reflected at the interfaces by 0.44%. The reflection ray is not intended by the design, and if the ray is reflected a plurality of times and then is incident on an image sensing element, the ray may result in a defect, such as a flare that causes reduction in contrast, or formation of a ghost image that is visually recognized as vivid light dots or light lines. To restrict the interface reflection, it is effective to control the difference between the refractive indices of the glass and resin to be within 0.1. However, if a ray is incident at an angle, a reflection characteristic when the ray is incident from a high-refractive-index medium on a low-refractive-index medium is different from a refraction characteristic when the ray is incident from the low-refractive-index medium on the high-refractive-index medium. If the ray is incident from the high-refractive-index medium on the low-refractive-index medium at an angle larger than the critical angle, the ray is not refracted but totally reflected. In contrast, if the ray is incident from the low-refractive-index medium on the high-refractive-index medium, the critical angle is not present, and hence the total reflection does not occur. That is, in order to restrict the reflection at the interfaces, the condition that the difference in refractive index is set within 0.1 is not sufficient.
Further, if the difference between the Abbe numbers of the resins used for the junction type compound lens is increased, the chromatic aberration is effectively corrected. However, it is known that the relationship between the refractive index and the Abbe number of resin generally has a linear relationship. If the difference in Abbe number is increased to correct the chromatic aberration, the difference between the refractive indices of the two resins is increased. If the difference between the refractive indices is increased, the reflection at the interfaces between the glass and resin is increased as described above, and an optical defect, such as a flare or a ghost image, is generated. Therefore, focusing only on the Abbe number to correct the chromatic aberration is insufficient for the technique of increasing optical performance.